[Distributed] remoteCallVoid is a new requirement in Swift 6.0

And it is handled compatibly because all existing implementations must
have already implemented it before -- it was an ad-hoc requirement
before Swift 6.0, and now it has become a real requirement - the same as
all the other ad-hoc requirements relying on the Serialization
Requirement

Author: ktoso

stdlib/public/Distributed/DistributedActorSystem.swift

@@ -357,6 +357,7 @@ public protocol DistributedActorSystem<SerializationRequirement>: Sendable {
   /// ## Errors
   /// This method is allowed to throw because of underlying transport or serialization errors,
   /// as well as by re-throwing the error received from the remote callee (if able to).
+  @available(SwiftStdlib 6.0, *)
   func remoteCallVoid<Act, Err>(
       on actor: Act,
       target: RemoteCallTarget,